Document DE 34 09 705 A1, which is incorporated by reference, discloses a method for providing feedback about disruptions or faults during a braking process during which a fault reaction-actuating device acts on the brake pedal travel simulator in such a way that a change in the brake pedal characteristic predefined by the brake pedal travel simulator occurs and remains until the provided function of the by-wire system is available again. Furthermore, it is proposed to change the brake pedal characteristic preferably in such a way that rhythmic pulsating of the brake pedal occurs. For this purpose, an electromagnetic actuator which is actuated with the pulsating signal of an electronic oscillatory circuit is to engage with a locking projection in a mechanism with which the simulator characteristic can be modified. The locking projection is designed to enter into a locking operative connection with a sliding piece on which a spring is supported by means of which part of the pedal force is transmitted. A technical teaching such as how pulsating of the brake pedal is to be generated by means of periodic activation of the locking is not provided. In order to permit periodic pedal movement, once an additional pedal travel has been enabled it must be capable of being reset again. A corresponding pedal actuating device and its supply with actuating energy would accordingly still have to be added.
German laid-open patent application DE 10 2006 026 873 A1, which is incorporated by reference, describes an activation device for a by-wire brake system. In order to provide the driver with haptic feedback about a relatively low coefficient of adhesion or friction at the vehicle wheels or a start of ABS control, the brake pedal axle is provided with an electrical, hydraulic or pneumatic rotary drive which generates pulsations at the brake pedal. In this context, the amplitude of the pulsations can be changed in a range between approximately 0 mm and approximately 5.0 mm, and the frequency thereof between approximately 1 Hz and approximately 10 Hz.
Finally, document DE 10 2004 040 616 A1, which is incorporated by reference, discloses a method for providing haptic information to the driver of a motor vehicle, which is equipped with a brake-by-wire brake system, about the operating state of the brake system, in which a brake pedal characteristic is generated in the form of a functional relationship between the brake pedal opposing force and the brake pedal travel, by a simulator, and is modified as a function of the operating state, as well as a device for carrying out the method. An opposing force, which counteracts the activation force applied to the brake pedal by the driver's foot is generated by an active pedal travel simulator, specifically in such a way that the driver senses this as a limitation of the comfort and/or functioning of the brake system.
Such limitation of the comfort and a sensed limitation of functioning are not acceptable if the driver is to be informed about the operating state of an intact brake system.